On cold start the engine has a knocking sound & sometimes shakes as well. After it warms up, sounds go away & runs fine. Sounds like the engine isn't getting oil but I have had the oil pump replaced & also replaced the camshaft synchronizer. Have had other issues & had it checked for a blown head gasket which checked out ok. Have replaced plugs & plug wires. I've heard others talk of Piston slap & Rod knock. What are these & what is the difference? It was suggested that I use a heavier oil, does this sound right or not? Thanks for any insight.

Definitely sounds like a piston slap. A piston slap occurs when the piston has too much clearance between itself and the cylinder bore. It mainly is just caused from normal engine wear. I'm pretty sure its not too bad for your engine, just don't be too hard on it while its making the sound.

When the engine is cold the clearance between the two is greatest because the piston has not expanded from heat, but once it heats up the piston will expand faster since it is aluminum and the cylinder is iron. That would explain why it only happens for cold starts.

There is one way I've heard of testing for it though. Turn your engine on when its cold, listen for about ten seconds and memorize the pattern. Quickly shut it off, take out the spark plugs and put a squirt or two of motor oil into each cylinder. Put the plugs back and give the oil a second to reach the piston. Turn the engine on again, if it is a piston slap the sound should be greatly reduced or gone for at least 20-30 seconds.

use lucas oil additive on your next change, see if that helps, i believe what you are expierencing is called a dry start, not only is is bad sounding, it is the leading cause of engine wear... my2cents

Lucas works but its a thick additive and can cause cold start chatter too. I suggest spending few extra bucks on good oil filters, cheapo's can cause it as well.. if you have over 150k miles. I suggest trying lucas (use full quart) but subtract a quart of oil when refilling.. if it doesnt help or stop it, recommend using 2qts 30w and 3qts of 10w30 without Lucas.